Heavy-duty rock bits are employed for drilling wells in subterranean formations for oil, gas, geothermal steam, and the like. Such bits have a body connected to a drill string and a plurality, typically three, of hollow cutter cones mounted on the body for drilling rock formations. The cutter cones are mounted on steel journals or pins integral with the bit body at its lower end. In use, the drill string and bit body are rotated in the bore hole, and each cone is caused to rotate on its respective journal as the cone contacts the bottom of the bore hole being drilled.
While such a rock bit is used in hard, tough formations, high pressures and temperatures are encountered. The total useful life of a rock bit in such severe environments is in the order of 20 to 200 hours for bits in sizes of about 61/2 to 121/4 inch diameter at depths of about 5000 to 20,000 feet. Useful lifetimes of about 65 to 150 hours are typical.
When a rock bit wears out or fails as a bore hole is being drilled, it is necessary to withdraw the drill string for replacing the bit. The time required to make a round trip for replacing a bit is essentially lost from drilling operations. This time can become a significant portion of the total time for completing a well, particularly as the well depths become great. It is therefore quite desirable to maximize the lifetime of a drill bit in a rock formation. Prolonging the time of drilling minimizes the lost time in "round tripping" the drill string for replacing bits.
Replacement of a drill bit can be required for a number of reasons, including wearing out or breakage of the structure contacting the rock formation. One reason for replacing the rock bits includes failure or severe wear of the journal bearings on which the cutter cones are mounted. These bearings are subject to very high pressure drilling loads, high hydrostatic pressures in the hole being drilled, and high temperatures due to drilling, as well as elevated temperatures in the formation being drilled. Considerable development work has been conducted over the years to produce bearing structures and to employ materials that minimize wear and failure of such bearings.
The journal bearings are lubricated with grease adapted to such severe conditions. Such lubricants are a critical element in the life of a rock bit and considerable work has been done to improve such greases. Lubrication failure can sometimes be attributed to misfit of bearings or O-ring seal failure, as well as problems with a grease.
Pressure and temperature conditions in a rock bit can vary with time as the rock bit is used. For example, when a "joint" of pipe is added to the drill string, weight on the bit can be relieved and slight flexing can occur. Such variations can result in "pumping" of the grease through O-ring seals, leading to loss of grease or introduction of foreign materials, such as drilling mud, that can damage bearing surfaces. It is desirable to provide a seal with sufficient resilience to resist such changing conditions, and to retain that resilience over a long period of time under adverse conditions.
One of the consistent problems in rock bits is the inconsistency of lifetime. Sometimes bits last for long periods, whereas bits which are apparently identical operated under similar conditions may fail with a short lifetime. One cause of erratic lifetime is failure of the bearings. Bearing failure can often be traced to failure of the dynamic seal that retains lubricant in the bearing. Lubricant may be lost if the seal fails, or abrasive particles of rock may work their way into the bearing surfaces, causing excessive wear.
Rock bit O-rings are being called on to perform service in environments which are extremely harsh. Some modern bits are being run at high rotational speeds, for example, 250 RPM for a 77/8 inch diameter bit, with some 171/2 inch diameter bits being operated at speeds up to 325 RPM. Such high rotational speeds impose high surface speeds on the dynamic O-ring seals, sometimes more than 100 meters per minute and often more than 50 meters per minute. This exacerbates the problems of elevated temperature due to frictional heating and slow dissipation of that heat. The life of the O-ring may be significantly degraded by the resultant high temperatures due to friction (as well as elevated temperature in the well bore) and abrasion.
It is therefore desirable to provide a consistently reliable dynamic O-ring seal for maintaining the lubricant within rock bits over a long useful life at high operating speeds. The O-ring should be resistant to crude gasoline and other chemical compositions found within oil wells, have high heat resistance and should not readily deform under changing load to allow leakage of the grease from within the bit or intrusion of abrasive drilling mud into the bearing.